7.2 Populations

Question 1

Populations

Answer 1

Species exist as one or more populations.

A population is a group of organisms of the same species
occupying a particular space at a particular time that can potentially
interbreed.

Question 2

Gene pool

Answer 2

A gene pool is all the alleles in a population (NOT all the genes in a population). You should recognise that the gene pool exists over deep time - only extinction of the population ends it. With successive generations, the frequencies of the alleles in the gene pool change - advantageous alleles become more common, while disadvantageous alleles are selected against.

Allele frequencies are the relative proportions of the different alleles of genes in the population. Frequencies are always represented as numbers between 1 and 0.

Question 3

The Hardy–Weinberg principle

Answer 3

The Hardy–Weinberg principle provides a mathematical model,
which predicts that allele frequencies will not change from
generation to generation.

The frequency of alleles, genotypes and phenotypes in a
population can be calculated using the Hardy–Weinberg equation:
p + q = 1
p² + 2pq + q² = 1
where p is the frequency of one (usually the dominant) allele and q is the frequency of the other (usually recessive) allele of the gene.

p + q = 1
states that all the alleles in a population are either dominant or recessive: p (the dominant) and q (the recessive) add up to 100% of the alleles. In other words p + q = 1 is the allele frequencies.

p² + 2pq + q² = 1
states that all the genotypes are either homozygous dominant (p²), heterozygous (2pq) (2 because there are twice as many of these), or homozygous recessive (q²). In other words p² + 2pq + q² = 1 is the genotype frequencies.

q² is the genotype that expresses the recessive trait.
p² + 2pq are the genotypes that express the dominant trait.

The conditions under which the principle applies:
No mutations
No selection (all alleles equally fit)
Random mating
Large population size
No gene flow (immigration/emigration)

DON’T FORGET if you are told the number of organisms expressing the dominant phenotype, you are being told p² + 2pq. Take this frequency away from 1 to find q².